Methods of thermally treating tissue are frequently used for a wide variety of medical purposes. For example, cryoablation, by which tissue may be destroyed, may be used to treat cardiac arrhythmia, to ablate tumors in the treatment of cancer, and for dermatological and obstetric procedures. Further, cooling tissue to sub-lethal temperatures is commonly used in electrophysiology studies.
Regardless of the tissue being treated, the permanency of the lesion created by a cryoablation procedure is of the utmost importance. The target tissue must be completely and permanently affected, which prevents the recurrence of the condition being treated. In the treatment of certain forms of arrhythmia, including atrial fibrillation (AF), permanent electrical conduction blocks must be created at specific locations in the heart. Therefore, continuous, transmural lesions must be created, ensuring that all myocardial cells in the target tissue are destroyed. Specifically, myocytes are killed acutely by cold-induced injury through several mechanisms that can include cell membrane rupture due to ice formation, osmotic imbalance, dehydration, damage to the mitochondria, and delayed apoptotic processes.
The type and severity of the damage to tissue cells is influenced by several parameters of the treatment process. In a cryoablation procedure, for example, these parameters may include duration of the freeze, treatment temperature, cooling and thawing rate, and the number of freeze-thaw-freeze cycles. In fact, extending the thawing phase of a cryoablation procedure by creating a temperature plateau at a mildly cold temperature, between approximately −20° C. and approximately −25° C., may result in more complete cell destruction and, therefore, a reduced likelihood of reconduction. Additionally, maintaining the treatment element in this temperature range may reduce the occurrence of collateral damage by preventing the freeze zone from penetrating too deeply within the tissue.
Current cardiac cryoablation systems operate at a controlled refrigerant flow to the treatment device, resulting in an operating temperature that is the lowest achievable in the given conditions. That is, the operator does not have the means to control the minimum temperature or the cooling and thawing rates. The only parameters that can be controlled are the duration of the freeze and the number of freeze-thaw-freeze cycles.
It is therefore desirable to provide a method and system by which parameters of a cryoablation procedure may be fully controllable. For example, it is desirable to provide a method and system by which duration of the freeze, treatment temperature, cooling and thawing rates, and the number of freeze-thaw-freeze cycles may be controlled by the operator.